Materials generally exhibit a certain band gap related to the material's physical and electronic properties. A band gap is specific to each solid material, and may be defined as an energy range in which there exists no electron state for placement of electrons. The band gap is described herein in terms of the energy difference (in electron volts eV) between the valence band and the conduction band for a material. The lower the band gap, the easier it is to ionize a material, e.g., by removing an electron.
High band gap materials, particularly semiconductors, are useful in photoconductive semiconductor switch (PCSS) applications. How high the band gap must be before the material is considered a “high band gap” material depends on what application the material is being used for. Generally, a high band gap material can be considered any material having an electronic band gap larger than about 1.0 eV. However, in some application, materials with an electronic band gap of larger than about 2.0 eV or more may be considered high band gap materials.
In addition to a high band gap, materials that are useful as PCSS material have high critical electrical strength, high electron saturation velocity, high thermal conductivity, and low “on resistance” when excited by a laser, or other optical source, with the proper wavelength and power.
Conventional materials that are used in PCSS applications include 6H-silicon carbide (SiC), 4H—SiC, and 2H-gallium nitride (GaN). 6H—SiC and 2H—GaN have wide band-gaps (3.0-3.4 eV), high critical field strength (3.0-3.5 MV/cm) and high-saturated electron velocity (2.0-2.5×107 cm/s). These material properties make semi-insulating 6H—SiC and GaN attractive semiconductor materials for PCSS applications. Previous SiC PCSS work used high resistivity, low impurity SiC polytypes and focused on lateral geometry surface switches that used above band-gap wavelengths of light to trigger the switches.
These materials must be obtained somehow, either through purchase or creation. The problem with purchasing these materials for PCSS applications is that these materials are very expensive and relatively difficult to obtain. Moreover, if these materials are grown, other problems exist. In order to grow a crystal to sufficient size so that it can be cut into the source material for a specific PCSS application requires a lengthy crystal growing process. In addition, the crystal growing process is difficult to control, and frequently leads to crystal boules with significant imperfections, such as “pipes,” inclusions, impurities, and/or other defects, which reduce the useful yield of the boule itself and yields a final crystal product with less than desirable performance characteristics, especially in optical applications, such as PCSS applications.
Since it is difficult to find all of the desired properties in a single material which can be used in PCSS application in a cost efficient way, it would be desirable to have methods to make materials that can be used in PCSS applications and/or to have additional materials capable of being used in PCSS applications that can be manufactured and/or produced more inexpensively and precisely than conventionally used materials.